1. Field of the Invention
This invention relates generally to the art of processing and treating molten metal, molten alloys, molten salts, or any other molten materials (hereinafter collectively referred to as “molten materials”).
2. Discussion of the Art
In the course of processing molten materials, it is often necessary to transfer the molten materials from one vessel to another or to circulate the molten materials within a vessel. Pumps for processing molten materials are commonly used for these purposes. The pumps can also be used for other purposes, such as to inject purifying gases into the molten materials being pumped. A variety of pumps as described are available from Metallic Systems Co., L.P., 31935 Aurora Road, Solon, Ohio 44139.
In the case where a molten material is melted in a reverbatory furnace, the furnace is typically provided with an external well in which a pump is disposed. When it is desired to remove molten materials from the vessel, a transfer pump is used. When it is desired to circulate molten materials within the vessel, a circulation pump is used. When it is desired to modify molten materials disposed within the vessel, a gas injection pump is used.
In each of these pumps, a rotatable impeller is disposed within a cavity or housing of a base member that is immersed in a molten material. Upon rotation of the impeller, the molten material is pumped through an outlet or discharge opening and processed in a manner dependent upon the type of pump being used. The impeller itself is supported for rotation in the base member by means of a rotatable shaft. The shaft is rotated by a motor provided at the shaft's upper end. Several support posts extend from a motor support platform to the base member for supporting and suspending the base member within the molten material. In addition, risers may extend upward from the base member for providing a path or channel for the molten materials to exit through.
Although pumps of the foregoing type have been in effective operation for several years, they still suffer from a variety of shortcomings. For example, graphite or ceramic (i.e. refractory materials) are typically the materials used for constructing many of the components of pumps used for processing molten materials because of its low cost, relative inertness to corrosion, and its thermal shock resistance. Although graphite has advantages when used for certain components of molten material pumps, it is not the most advantageous material to be used for complicated shapes and mechanically stressed components.
Rather, it is preferable to make these types of components, e.g. support posts, risers and rotating shafts, from a metallic material, such as iron based alloys or steel, since metallic materials are considerably stronger per pound than graphite. The problem with using these materials is that the base member and impeller are typically constructed from graphite (due to its wear characteristics) and it is difficult to maintain a connection between metallic and graphite components. Such a difficulty arises because of the differences in thermal expansion experienced by these materials. Accordingly, bolts and conventional fasteners are generally not feasible connecting mechanisms. Moreover, the simplest connection would be for the metallic shaft to include a circular threaded male member which is configured to be received within a threaded female member of the graphite impeller.
A second problem arises in connection with attaching a metallic shaft to a graphite impeller. Particularly, because graphite is a relatively weak material, the graphite threads of the impeller are easily stripped upon shaft rotation.
Even when the two components to be connected are of the same material, such as the base and riser of a pump for processing molten zinc or molten magnesium, there are connection problems. For example, the use of bolts and fasteners as a connecting mechanism do not provide optimal strength.
A third problem with known molten material pumps is that the pump components are often manufactured with clearances, tolerances, etc. which permit molten materials to escape from the cavity or housing of the base member. Because the pressure outside the base member is much less than that within the base member, the molten materials naturally gravitate toward the crevices created by the clearances and tolerances. Accordingly it is difficult to maintain an effective seal within the base member's housing.
A fourth problem associated with the foregoing molten material pumps is that the shafts of these pumps have a tendency to grow in length at elevated temperatures due to thermal expansion. The increased length often pushes the pump out of alignment. Similarly, the riser can bend or move during operation and push the pump out of alignment.
Accordingly, a need exists in the art of processing molten materials to provide a molten material pump having an improved (1) sealing assembly, (2) self-aligning shaft, (3) shaft-to-impeller connection, (4) impeller design, and (5) connection assembly for other pump components.